Move download functionality to CKAN.Net

Also, tests! Aww yis.
Implemeted `ckan update` and `ckan available`.

* Downloads CKAN metadata from github.
* Closes #33 (index all mods)
* Closes #34 (ckan update)

NOTE: This changes the format of the CKAN registry. You'll need to
manually delete it from your KSP/CKAN directory, lest you get
weird-ass errors.
Merge pull request #41 from pjf/ckan_update

Implements `ckan update` and `ckan available`.